Web/iOS/Android: Whether you're a student in school or a lifelong learner, using flash cards can help you retain information. StudyBlue is a crowd-sourced collection of these study aids.
We've mentioned StudyBlue before, but it's changed (for the better) since then. Now you can sign up without having to go to a university that's connected to the app. In addition to making your own flash cards and testing yourself on them, you can access others' flash cards on topics ranging from Algebra to Zoology.
The free version gives you suggested flashcard sets for the interests you add, and a paid version $US7 a month (paid yearly) gives you full access to all 250+ million decks.